one more question for Keet. I noticed that the buds on the 3D model of the AI rose turned out to have a different transparency. The stem and leaves are opaque, and the flower is a little transparent! There is nothing about this in the MTL file! Is it even legal? How can that be? The texture is uniform, not divided into parts. Unbelievable.

That is easy to explain. Transparency in an image is created by adding an alpha channel and then define a specific color as transparent. Exactly the same as how you create a transparent background. I think you can even define mutiple colors as semi-transparent, but I'm not sure. The rose petels that are semi-transparent have that transparent color mixed in.